When drilling deep bores, such as in the oil and gas exploration and production industry, it is now conventional to select and control the inclination and azimuth of a bore as it is drilled. One technique employed to achieve this is known as directional drilling; this typically involves the use of a “bent” sub towards the end of a drilling string. The axis of the bent sub includes a small deviation, perhaps of 0.5 degrees, and thus has the effect of directing the drill bit away from bore axis. When the drill string is rotated from surface, the bent sub is rotated and the effect of the deviation is negated. However, if a downhole motor is utilised to rotate the drill bit, and the bent sub is held at a desired orientation, the drill bit will deviate from the bore axis.
It is widely recognised that better drilling rates are achieved when a drill string is rotated from surface, and that there are many disadvantages associated with drilling operations in which the drill bit is rotated by a downhole motor mounted on a non-rotating string. Accordingly, there have been numerous proposals for “rotary steerable” systems, that is drilling arrangements which allow the drilling direction to be controlled while still permitting the drill string and bit to be rotated from surface. The applicant has made a number of proposals in respect of such systems, as described in UK Patent GB2382361B, International Patent Application WO 03/102353, U.S. patent application Ser. No. 10/785,456, and UK Patent Application GB2394235A, the disclosures of which are incorporated herein by reference.
It is among the objectives of embodiments of the present invention to provide directional drilling apparatus and methods which achieve this aim.